Optical data storage media include a broad range of materials and signal mechanisms, and include media in which recording takes place before media production ("read-only"). media on which data can be directly recorded and becomes permanently fixed ("write-once"), and media which can be both recorded upon and erased and re-recorded ("erasable"). Optical signals fall within three general categories: reflective, transmissive and absorptive. These signals may be produced in a variety of ways, including bump-forming media in which pits (or bumps) are formed in certain layers of the media; optical density change materials (such as photographic films, photoresists and photopolymers, which undergo optical density changes upon absorption of light); phase-change materials (undergoing a transition from a crystalline to an amorphous state or vice versa upon absorption of light); magneto-optical materials (where signals are recorded by localized heating under a magnetic field to change the direction of magnetization); and ablative thin films (where the recorded pattern induces light amplitude modulation).
Many of these techniques are examples of thermo-optical recording, in which light from a laser is focused to a small, usually diffraction limited spot at a specified depth in the medium. The energy from the focused light heats the spot and effects the change which functions as data storage.
The construction of the medium will vary depending on the type of signal to be recorded on it or built into its structure during fabrication. Optical media in general are of multilayer construction. Ablative media form a single layer of polymer which may be coated with a metal reflective layer. Bump-forming media have a multi-layer construction, also including a metal reflective layer.
Examples of optical data storage media which include layers of differing absorptivity are those described in European Patent Application Publication No. 136070, published on Apr. 3, 1985, entitled "Erasable Optical Data Storage Medium and Method and Apparatus for Recording Data on the Medium" (Optical Data, Inc.); and U.S. patent application Ser. No. 153,288, filed concurrently herewith, inventors B. Clark. J. Finegan and R. Guerra, having the same assignee as named herein, entitled "Optical Data Storage Media for Substrate Incident Recording." In such media, binary optical data appear as pits or bumps in an otherwise flat reflecting surface, which may be either a partially reflecting interface between two layers of different refractive indices, or a fully reflecting surface such as a metallic film.
Applications of dye-polymer technology to recording media are intended for a playback on optical players such as Compact Discs (R) audio players, Laser-vision (R) video players and CD-ROM computer systems. These applications require that the media have a reflectance of greater than 70at the wavelength of the playback laser, the wavelength typically being 780 nm. Commercially produced pre-recorded discs meet this requirement by the use of aluminum, or, occasionally, gold vacuum deposited or sputtered over the recorded information. Experimental recordable dye-polymer media have also used aluminum or gold, whether the principle of operation is ablative or bump-forming. These two types of dye-polymer media, however, have different requirements with respect to the metal reflective layer.
In the ablative media, when the polymer layer is removed by the action of the recording laser, the metal layer must be removed over exactly the same area as the polymer. This is necessary to insure that the contrast between the reflective unrecorded and the recorded mark is great enough to be detected by the playback system. By removing the metal layer with the polymer layer, the change in contrast between the recorded and unrecorded areas is more easily detectable.
Difficulties arise with the use of aluminum or gold because under typical conditions, an extremely high temperature is required to remove the metal reflective layer along with the polymer layer. It is difficult to control melting of the reflective layer. In addition, if the metal layer is removed, blasting will result in ragged edges.
In bump-forming media, when the polymer layer is deformed, the metal layer must be deformed simultaneously by the forces created in the two-layer structure by the recording laser. The deformation must be great enough to permit the bump to be detected by the playback system. Again, aluminum and gold are difficult to deform in this manner.
Computer models have shown that temperatures in the polymer layers at the points of focus of the recording laser under typical recording conditions are around 800.degree. C. Since the reflective layer is a small distance from the focal point, of course, the temperature in the reflective layer is somewhat lower. The metals normally used in reflective layers of the prior art (where they are used in read-only media), however, are not sufficiently responsive at these temperatures, particularly at the thicknesses needed to achieve good reflectivity. One must therefore use beams of higher intensity to achieve sufficient energy in the reflective layer to record a mark.
Because the semi-conductor laser diodes typically used in optical recorders have power limitations and an operating life highly dependent upon the power generated during operation, it is highly desirable to reduce the power needs of the media.